gen-briefly-explain-process-capability-index-cpk-and-one-common-ffb2

Briefly explain process capability index (Cpk) and one common misconception about it.

## Process Capability Index (Cpk) **Cpk** measures how well a process fits within specification limits, accounting for both process variation AND centering. It's calculated as the minimum distance from the process mean to either specification limit, divided by half the process spread (3 standard deviations). **Formula:** Cpk = min[(USL - μ)/(3σ), (μ - LSL)/(3σ)] - **Cpk ≥ 1.33**: Generally considered capable - **Cpk < 1.0**: Process produces defects outside specifications ## Common Misconception **"A high Cpk means my process is performing well"** This is misleading because: - **Cpk assumes the process is stable and in statistical control** - If your process has special causes, shifts, or trends, Cpk calculations are meaningless - You could have a high Cpk value calculated from unstable data that doesn't represent actual process performance **Bottom line:** Always verify process stability with a control chart BEFORE calculating Cpk. Otherwise, you're measuring the capability of a process that doesn't consistently exist.